Actually, you can't use a ForwardResolution to go to another site. A Forward is an internal concept within the Servlet container. What you can do is a RedirectResolution, and you can use that to send a normal GET query to another site, including query parameters.
http://example.com/action?search=thing
But, that will be a GET, not a POST.
Redirect works because it sends the URL to the browser, and the browser then resubmits it to the destination site.
The only way to send a POST is to send a populated HTML form to the browser, with the action parameter pointing to the new site and method="POST", and then use a little bit of Javascript to automatically submit the form once the page is loaded. For a small form filled with hidden fields, this is quite fast, most users won't even see it happen, but it does require javascript to be enabled in their browser.